Diethylstilbestrol (DES), a drug first synthesized in 1938, was administered to several million pregnant women in the U.S. and Europe for the prevention of spontaneous abortion and premature delivery. In 1971, Herbst reported a strong association between DES use in pregnancy and the occurrence of vaginal clear cell adenocarcinoma (CCA) in exposed female offspring. Animal models have demonstrated a range of DES effects on offspring exposed in utero, including reproductive dysfunction, immune system changes, behavioral and sexual abnormalities, and increases in various reproductive cancer in males and females. NCI, in collaboration with five field centers, reassembled previously studied cohorts of DES-exposed and unexposed mothers, daughters and sons, and identified subjects with documented exposure status who had not been studied previously, through familial links within the cohorts. Standardized baseline questionnaires were mailed to cohort members to ascertain the risk of cancer and other disorders. Pathology reports were collected for reported cancers and preneoplastic conditions. Three separate phases of follow-up have been conducted. The purpose of this study is to continue the follow-up, by means of mailed questionnaires and medical record collection, which was begun during the the first phase of the study. Concern has arisen that DES-exposed daughters may be at higher risk of breast cancer. Exposure to high levels of endogenous estrogen in utero has been hypothesized to increase the risk of breast cancer and DESis a potent estrogen. Cancer risk in the sons will also continue to be assessed, especially for increased risks of prostate cancer. since the offspring who were exposed to DES in utero are currently reaching their late forties, when cancer rates begin to rise, it is important to continue the follow-up of these cohorts to determine if there are long-term increases in cancer risk.